This invention relates to a flexible non-conductive sealing member used in an electrolytic cell particularly of the diaphragm type for the electrolysis of brines. More specifically this invention relates to a flexible vulcanized sheet to provide a seal and cover for the base of such an electrolytic cell. This invention even more specifically relates to such a sheet having improved sealing properties.
Diaphragm-type cells conventionally have included an outer steel shell or can either of a cylindrical or rectangular configuration which supports a foraminous or perforated metallic cathode constituting the cathode assembly. A fluid permeable diaphragm overlays the cathode to permit the brine solution contained in the anode compartment to flow or percolate through the diaphragm and cathode into the cathode chamber. The diaphragms and cathodes are usually arranged vertically but may also be disposed horizontally. The anodes conventionally take the form of flat vertically disposed blades of graphite which have been inserted into slots formed by a plurality of conductive copper metal grids which are mounted on the cell base. The grid members and the anodes are electrically connected to each other and are usually secured through the base by an electrically conductive bonding layer of lead. An electrically insulating layer or coating of a material such as asphalt is then applied over the electrically conductive bonding layer and a layer of concrete is finally applied over the asphalt layer to complete the base construction. A detailed description of an electrolytic cell of the type described is given in U.S. Pat. No. 2,987,463 issued on June 6, 1971, to J. C. Baker et al. In addition, a complete discussion of the construction and operation of a diaphragm type cell can be found in the Encyclopedia of Chemical Technology, Second Edition, Vol. 1 (1963) on Pages 681-687.
Recently the conventional graphite anodes have been replaced by the development of dimensionally stable anodes. Consequently, it has become necessary to provide diaphragm-type electrolytic cells having a cell base of a more simple construction suitable for use with dimensionally stable anodes. U.S. Pat. No. 3,591,483 issued July 6, 1971, to R. E. Loftfield et al discloses a diaphragm-type electrolytic cell having improved cell base constructions in which a single sheet of at least one electrically non-conductive material such as neoprene rubber covers the entire cell base and serves to provide a compressible seal between the anodes and the cell base and between the cell base and the brine solution contained in the cell can. The sheet and the cell base are provided with aligned holes for the receipt of anode risers which extend through these holes in the non-conductive sheet and the cell base and are fastened on the bottom of the cell base by suitable means such as a nut. Each riser is provided with a flange or collar which upon tightening of the nut, forms a hydraulic seal with the non-conductive sheet, thereby preventing leakage of the brine solution or solution of electrolyte onto the cell base. It has also been the practice in at least some instances for added protection against the effects of the brine solution to use a corrosion resistant grease-like sealing material between the neoprene rubber sheet and the area of the cell base disposed beneath the collar and between the neoprene rubber sheet and the area around and under the collar.
Although neoprene rubber has proven to be a suitable material for providing a seal and cover for the conductive base for limited periods of time, it has not been completely satisfactory over long periods of time. Often the elastomeric sheet has become hard and brittle due to the relatively high temperatures to which the sheet is subjected. These temperatures range from about 180.degree. to 190.degree. F (82.degree. to 88.degree. C) on the face of the sheet in contact with the brine or electrolyte solution and from about 180.degree. to 250.degree. F (82.degree. to 121.degree. C) on the opposite face of the sheet which is in contact with the conductive copper or aluminum base of the cell.
Because of this, a flexible sheet comprised of a vulcanized blend of heat resistant polymers has been developed for improved resistance to heat over long periods of time. This sheet is disclosed in our application Ser. No. 371,323 issued as U.S. Pat. No. 3,857,775 on Dec. 31, 1974.
It is also well known that the diaphragm chlorine cell units fluctuate in temperature from a normal operating temperature of approximately 180.degree. F (82.degree. C) which may reach as high as 250.degree. F (121.degree. C) on the base plate, to ambient and back to operating temperature again. Due to this change or fluctuation in temperature it has been discovered that the seal between the anode riser and the rubber sheet or gasket may be broken which results in the seepage of the brine solution onto the conductive base of the cell and causes its rapid corrosion and deterioration. It is well recognized in this regard that even a minute amount of brine which is allowed to leak through the broken seal onto the copper or aluminum cell base will continue to corrode the metal part to ultimate failure even though the rubber gasket has become resealed as a result of a further temperature change. Consequently, in order for the sheet to perform satisfactorily for the life of the cell it is imperative that the seal be maintained at all times regardless of temperature change or other occurances during the life of the cell.
Accordingly, although it is, of course, important that the diaphragm sheet be formulated for heat resistance and chemical resistance, it is believed that sealability of the sheet will be optimized if the polymeric material of the sheet is compounded for optimum resistance to compression set as well. The sheet after long periods of exposure to heat therefore should have a relatively flexible, resilient, rubbery nature and also exhibit a low initial compression set with a retention of a relatively low set. In this way, it is anticipated that the low compression set characteristics (high resistance to compression set) will compensate for the changes in temperature and the compression seal between the outer upwardly facing surface of the sheet and the flange of the anode riser can be maintained continuously for the life of the sheet.
Other type sealing members in addition to the flat sheet may also be used in an electrolytic cell to provide a compressive seal between rigid members of the cell and are subject to these same conditions and resulting problems.